The invention pertains to a process for automatically controlling the rail pressure in an internal combustion engine with a common rail system during a starting operation.
To achieve high injection quality and low pollutant emissions, the rail pressure in an internal combustion engine is automatically controlled by a common rail system. A closed-loop control circuit is known from DE 103 30 466 B3, in which the actual rail pressure is calculated from the raw rail pressure measurements and compared with the nominal rail pressure, which is the command variable. From the resulting control deviation, an automatic pressure controller calculates a volume flow rate as the correcting variable, which is then limited and converted to a pulse width modulation (PWM) signal. The PWM signal is sent to the magnetic coil of a suction throttle. This suction throttle influences the flow delivered by a low-pressure pump to a high-pressure pump, which then conveys the fuel to the rail while increasing its pressure. In this closed-loop control circuit, the two pumps, the suction throttle, and the rail correspond to the controlled system. The unpublished German patent application with the official file no. DE 10 2006 049 266.8 discloses the same closed-loop control circuit with the more precise statement that the volume flow rate is converted by the use of a characteristic pump curve to a nominal electric current, which then serves as the input variable for the PWM calculation.
In practice, the following problem can occur in this automatic pressure control circuit during the starting operation:
To calculate the PWM signal, the nominal electric current is multiplied by the ohmic resistance of the suction throttle coil and the (electric) line. The suction throttle is driven with negative logic; that is, the throttle is open when no current is passing through it. When the suction throttle is completely open, the volume flow rate delivered by the low-pressure pump arrives unthrottled at the high-pressure pump. When current is sent to the suction throttle, it closes the fuel line. To guarantee a reliable drive to zero, that is, a complete closing of the fuel line, it must be assumed that the ohmic resistance of the suction throttle coil and the (electric) line is at its maximum. The maximum resistance value is obtained at the maximum temperature of the suction throttle. In a permissible temperature range between −20° C. to 120° C., for example, the ohmic resistance of the suction throttle changes from about 2 ohms to 4 ohms, that is, by 100%. So that the high pressure can be reduced reliably to zero under all possible environmental conditions, the maximum fixed value of 4 ohms must be stored in the electronic control unit. At low temperatures, however, this leads to an improper calculation: because the actual resistance is low, the calculated PWM signal is too large. The suction throttle is thus driven toward the closed position. When the internal combustion engine is started in a cold environment, this has the result that, after the actual rail pressure has swung past the target value (negative control deviation), it swings back under the nominal rail pressure (positive control deviation) and continues to decrease until the pressure falls below the opening pressure of the injector nozzles. The internal combustion engine thus stops.
For the previously described automatic control circuit, this problem can be solved by providing another circuit to support the automatic rail pressure circuit, namely, a circuit for controlling the coil current as known from DE 10 2004 061 474 A1, for example. Because of the additional hardware, however, this solution is expensive.
Although DE 101 56 637 C1 describes a process for the open-loop and closed-loop control of the starting operation of an internal combustion engine, the goal of the process is to suppress pressure fluctuations by preventing an oscillation between open-loop and closed-loop control modes. No additional information can be derived from this source concerning the problem of interest described above.